1. Field of the Invention
This invention relates to a passive seat belt system of the type that after an occupant has sat in a seat, a slider fastened to one end of a restraining webbing runs on a guide rail provided on a vehicle body so as to automatically apply the webbing to the occupant. In particular, the present invention is concerned with a lock device for preventing movement of a slider toward an occupant-releasing side so as to ensure the restraint of an occupant when an acceleration/deceleration of at least a predetermined value is applied to an associated vehicle due to an impact such as a collision.
2. Description of the Related Art
In many of passive seat belt systems of the above sort, a rear portion of the guide rail, said portion being on an occupant-restraining side, is bent downwards with a view toward preventing the slider from moving forward or rearward in the event of a vehicular emergency such as a collision. If the vehicle turns over, a force is however exerted in a vertical direction to the slider so that the slider may be caused to move along the vertical portion of the guide rail and the restraint of the occupant may become incomplete, thereby failing to protect the occupant fully.
If the door should open due to an impact to the vehicle at the time of a collision, the occupant is released from his restraint by the webbing, leading to an accident such that the occupant may be thrown out of the vehicle. A variety of lock devices has therefore been proposed to lock the slider so that the occupant can be maintained in restraint even in such a case as vehicle over-turn or door opening at such a vehicular emergency.
It has conventionally been known, for example, to interlock a stopper, which is associated with an acceleration/deceleration sensing mechanism, with the slider so as to lock the slider directly (U.S. Pat. No. 4,223,915 issued Sept. 23, 1980 to Hideki Tanaka, et al.), to bring a stopper into engagement with a tape-like force-transmitting member adapted to drive the slider, thereby to lock the slider (U.S. Pat. No. 4,555,127 issued Nov. 16, 1985 to Osamu Kawai) or to provide a circuit for preventing a motor from rotating in a direction to move the slider in the occupant-releasing direction upon reception of an electrical signal from the acceleration/deceleration sensing mechanism (Japanese Patent Publication No. 20498/1984 published May 14, 1984 and naming Osamu Ichinose as a sole inventor).
The design that a stopper is brought into engagement with a slider to lock the slider directly is however accompanied by the following drawbacks. Namely, an acceleration/deceleration sensing mechanism must be provided around a B-pillar (an anchor point of a shoulder webbing) of a vehicle. The B-pillar hence extends inside the room of the vehicle, narrows down the room of the vehicle and gives a sort of squeezed feeling to the occupant. In addition, mounting holes are formed in the B-pillar, and a slot through which the stopper operates is also formed through the B-pillar. The strength of the B-pillar is thus reduced, leading to a reduction in the strength of the anchor on the vehicle.
The design that a stopper is brought into engagement with a force-transmitting member is not accompanied by such drawbacks, since the mounting position can be selected rather freely. Upon forward movement of a movable anchor, a slider is pushed forward at a forward end so that a front end switch is actuated to stop the rotation of a motor and the movement of the movable anchor is hence stopped. Until the motor stops subsequent to the actuation of the switch, the force-transmitting member is continuously pushed out of the motor owing to the influence of the inertia of the motor and the like. If it is designed to bear this pushing force 100% at the forward end, a considerable load is applied so that the force-transmitting member may be damaged, for example, at a portion where the movable anchor is pushed. In order to avoid such damages, it is practised to allow the force-transmitting member to flex so as to absorb the force. It is therefore not easy to set the position of installation of the stopper relative to the force-transmitting member which can still move to a certain extent in the longitudinal direction after it is stopped once as described above.
On the other hand, the provision of a control circuit for the control of a motor by an electrical detection signal results in a higher manufacturing cost compared with conventional mechanical means.